At a glance he assumes the best practically achievable efficacy for a photovoltaic panel in a solar farm is 10%:
http://www.withouthotair.com/c6/page_41.shtml
>If a breakthrough of solar technology occurs and the cost of photovoltaics
came down enough that we could deploy panels all over the countryside,
what is the maximum conceivable production? Well, if we covered 5% of
the UK with 10%-efficient panels, we’d have
This paragraph is titled "Fantasy time". So before I start the criticism, I would like to thank him for clearly debunking fantasies about hydro and geothermal, where aside from ground-source heat pumps and sparsely populated mountains, they are simply too small. Photovoltaics are the largest source of sustainable energy by far, so the results of an overall analysis will be heavily dependent on the treatment of solar panels.
At the same time, I think it's practical to assume that humans will aggressively innovate the properties and production of PV panels, because the potential value is so large. But I'm going to stick with existing technology. The Agua Caliente farm in Arizona:
https://www.solarfeeds.com/mag/solar-farms-in-the-usa/
uses CdTe panels from First Solar which are currently manufactured with about 16% efficiency:
https://www.firstsolar.com/-/media/First-Solar/Sustainabilit...
As such, describing solar farms as a fantasy, and upper bounding the efficiency at 10%, when there are existing installations built with solar panels at 16% efficiency, seems too pessimistic. Looking ahead to other technologies, perovskites, considered a low-cost option, were recently pushed to 25%:
https://www.pv-magazine.com/2021/04/06/unist-epfl-claim-25-6...
and Alta Devices demonstrated 29% efficiency with a GaAs thin-film before a buyout by a Chinese firm led to a class-action lawsuit filed by disgruntled employees:
https://en.m.wikipedia.org/wiki/Alta_Devices
I also take issue with the assertion on page 115:
http://www.withouthotair.com/c19/page_115.shtml
>most countries will be in the same boat as
Britain and will have no renewable energy to spare
A glance at a map will immediately show the viewer that Britain is one of the most poleward and densely populated countries in the world — a worst-case scenario for solar electricity. Even Japan has the benefit of sitting significantly further south.